A Computed Tomography (CT) device includes components such as a tube, a shape filter and a detector. The tube, the shape filter and the detector can rotate around the rotation axis of the gantry of the CT device, and the position of the rotation axis is fixed. Reference is made to FIG. 1 which illustrates a schematic diagram of a placement of an object to be scanned on a scanning plane. In the conventional CT scanning, the center line of the object to be scanned, i.e., of the human body, is at the rotation axis, in a case that the human body is not in the off-center position. Since the section of the human body is in a shape like an ellipse, the attenuation of X-ray emitted by the tube after passing through the human body is different in a case that there is no shape filter, and the noise of the rebuilt image may be not consistent. In view of this, a shape filter is provided between the tube and the human body, and the X-ray passes through the shape filter firstly and then passes through the human body. Under a certain projection angle of the X-ray, the intensity of each ray in the fan beam formed by X-rays passing through the shape filter is as shown by (1) in FIG. 1 in a case that the human body is not in an off-center position. It can be seen that, since the length that the central ray i1 travels in the shape filter is shortest, the intensity of the central ray i1 after passing through the shape filter is strongest; and the intensity of the rays on both sides of the central ray i1 gradually decreases as the rays leave farther from the central ray i1, because the length that the rays travels in the shape filter gradually increases as the rays leave farther from the central ray i1. Furthermore, the central ray i1 passes through the intersection of the center line of the human body and the scanning plane, i.e., the center point E of the human body, thus the path in which the ray with the strongest intensity (which aims at the center point of the human body) in the fan beam passes through the human body is the longest, and the path in which the ray with the weaker intensity on each side of the central ray i1 passes through the human body is shorter. Therefore, the intensity of the rays received by the detector is substantially the same, which is approximate to a horizontal, as shown by (2) in FIG. 1. In this case, more uniform dose of scanning is received by different parts of the human body, that is, the intensity of the ray which passes through the wider part of the section of the human body is stronger and the intensity of the ray which passes through the narrower part of the section of the human body is weaker, and thereby the total dose of scanning is reduced and the consistency of the image noise among various parts of a patient is improved (the noise is directly proportional to the intensity of the X-ray received by the detector).
Reference is made to FIG. 2 which illustrates another schematic diagram of a placement of an object to be scanned on a scanning plane. The center point of the object to be scanned, i.e., of the human body, is moved from the intersection of the rotation axis and the scanning line, i.e., the rotation center point E to the point F (in a perpendicular direction as shown in FIG. 2) in a case that the human body is in an off-center position. In this case, the ray i2 passing through the center point of the human body is not the ray i1 with the strongest intensity in fan beam (because the length of the line segment CD is not the shortest). Therefore, the dose of scanning received by different parts of the human body may not be uniform, and thereby the noise of the scan image is not consistent.